The Identity of the “Tully Monster” Remains a Mystery

In 1955, fossil collector Francis Tully discovered a strange fossil lying in a creek bed in Grundy County, Illinois. The fossil appeared to be some sort of worm, mollusk, or arthropod but did not resemble any other known life forms closely enough to be classified in any existing taxa.

A Tully Monster fossil.

According to University of Pennsylvania paleobiologist Lauren Sallan, many debates have been waged over the classification of this strange, stalk-eyed sea creature:

Initially it was published as a worm. There is a well-constructed argument that it is some kind of mollusc, like a sea slug. And there’s another very strong argument that it’s some kind of arthropod, similar to a lobster.

This 300-million-year-old creature, classified as Tullimonstrum gregarium or commonly referred to as the “Tully monster,” defied biological classification for decades. Until last year, that is. In 2016, a group of Yale biologists announced that they had finally conclusively identified the Tully Monster. According to their research, the Tully Monster was a vertebrate belonging to a class of animals that include modern lampreys and their extinct relatives. With this study, these researchers declared the Tully Monster case closed.

Not so fast, says a new study published in the journal Palaeontology. According to Sallan and her colleagues at U Penn, a further review of all the available evidence on the Tully Monster reveals it to be distinctly different from vertebrates such as lampreys:

It’s important to incorporate all lines of evidence when considering enigmatic fossils: anatomical, preservational and comparative. Applying that standard to the Tully monster argues strongly against a vertebrate identity. Not only does this creature have things that should not be preserved in vertebrates, it doesn’t have things that absolutely should be preserved.

Ouch! Take that, Yale. Ultimately, this new study does little to firmly identify the Tully Monster, but merely argues against the latest attempt at classifying it along with lampreys. Even though over 1,200 Tullimonstrum gregarium fossils have been discovered to date, the creature’s true identity remains a mystery. Could the Tully Monster be an ancient alien or merely some sort of evolutionary aberration? Whatever it might be, I’m happy just knowing that this creature remains an anomaly. A creepy, terrifying anomaly.